First, a minor change must be made to the freebsd-update code in order
for it to accept file names appearing in FreeBSD 9.0 which contain the '%'
and '@' characters; without this change, freebsd-update will error out
with the message "The update metadata is correctly signed, but failed an
integrity check".

# sed -i '' -e 's/=_/=%@_/' /usr/sbin/freebsd-update

Now freebsd-update can fetch bits belonging to 9.0-RC3. During this process
freebsd-update will ask for help in merging configuration files.

IMO it should be that FreeBSD 9.0 RC?-candidates has correct "freebsd-update" code...